If you go at a time that will overlap with fireworks at MK, you might have some trouble getting up. Other than that, anytime is good! When you go to the 2nd floor floor to ask if you can go up, if yes, they elevator you up to 15th floor, and there you will probably have option of bar or lounge area. My suggestion is take what's open or closest to open. You can get up and gaze at the cool view any time. The smaller your group the easier this is.
We've (only 2 of us) always been able to order off full menu in the bar or lounge. Any or all, and the servers are great. Just let them know what you're planning and they're on it. We've done just apps, just dessert, just wine, etc.
Have a great trip!